The End of Realism?

  1. I came across an article from 2007 talking about some experiemtns that are an extension of the experiments which originally tested for Bell's Inequality. These tests were supposedly designed to relax the locality requirements and test soley for realism. I'll quote a few pertinent sections:

    " Now physicists from Austria claim to have performed an experiment that rules out a broad class of hidden-variables theories that focus on realism -- giving the uneasy consequence that reality does not exist when we are not observing it "

    "Markus Aspelmeyer, Anton Zeilinger and colleagues from the University of Vienna, however, have now shown that realism is more of a problem than locality in the quantum world. They devised an experiment that violates a different inequality proposed by physicist Anthony Leggett in 2003 that relies only on realism, and relaxes the reliance on locality. "

    "They found that, just as in the realizations of Bell's thought experiment, Leggett's inequality is violated – thus stressing the quantum-mechanical assertion that reality does not exist when we're not observing it."

    I had some questions on the next quote here:

    "However, Alain Aspect, a physicist who performed the first Bell-type experiment in the 1980s, thinks the team's philosophical conclusions are subjective. "There are other types of non-local models that are not addressed by either Leggett's inequalities or the experiment," he said."

    I'm curious if anybody knows what other non-local models he is referencing? And, have these been able to be tested since 2007? What's the latest and greatest on all this?

    Either way, I agree with Alain Aspect when he says at the end of this article:

    "But, I rather share the view that such debates, and accompanying experiments such as those by [the Austrian team], allow us to look deeper into the mysteries of quantum mechanics."

    Entire article:

    http://physicsworld.com/cws/article/news/27640
     
  2. jcsd
  3. StevieTNZ

    StevieTNZ 1,047
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    The non-local hidden variable theory that wasn't ruled out is Bohm Mechanics. No experiment has ruled this theory out.
     
  4. I wasn't aware of that. I liked some of Bohm's ideas on a conceptual level, but I thought his theory either had mathematical inconsistencies or was incomplete?

    Perhaps, I was mistaken, though.
     
  5. StevieTNZ

    StevieTNZ 1,047
    Gold Member

    As far as I'm aware, that isn't the case. There are still papers being published that rule out Bohm Mechanics if you agree the experimenter has free-will, for example: http://arxiv.org/abs/1005.5173

    Also: http://arxiv.org/abs/1105.0133
     
  6. Interesting, I'll have to read through those. Thanks!
     
  7. bhobba

    bhobba 5,092
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    I beg to differ. Don't know about the free will thing mentioned later by StevieTNZ - but here is another paper:
    http://arxiv.org/pdf/quant-ph/0206196v1.pdf

    It doesn't seem to depend on free will.

    Thanks
    Bill
     
    Last edited: Jan 30, 2012
  8. StevieTNZ

    StevieTNZ 1,047
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    Thanks for that paper. I shall read it. Wasn't aware it existed.
     
  9. How does ruling out "hidden variables" mean there's no realism? Of course things stop at a certain point, that doesn't mean they aren't built upon in a logical manner...
    Unless they are confusing realism for detirminism?
    And if matter didn't stop after a certain point, wouldn't that imply there are infinite hidden variables and therefore it would be impossible to determine anything anyway?
    How did the conditions for life to form even come to be if there wasn't an existence before us?
     
    Last edited: Jan 30, 2012
  10. If you imply that dBB was ruled out experimentally, such implication seems dubious: Zeh (Physics Letters A, Volume 309, Issues 5–6, 31 March 2003, Pages 329–334 ) made this comment on the article you quoted: "Variants of Bohm's theory with photon trajectories instead of time-dependent Maxwell fields have recently been claimed to be in conflict with quantum theory and experiments [11]. While the analysis of these experiments appears doubtful, this modified Bohm theory—in contrast to the original one—seems to have never been proven equivalent to quantum theory."
     
  11. Ken G

    Ken G 3,566
    Gold Member

    You can't really expect an accurate rendition of what realism is from a pop sci article, that's the problem. The term means many different things in different contexts, but in this context, it seems to be associated with "independence from observation." In other words, realism is the belief that reality exists independently of how humans tinker with it and understand it. But that term never meant anything in science, because science is all about tinkering with nature and trying to understand it, and everything that science regards as real stems exactly from that process. Relevant here is the Bohr/Einstein debates, where Bohr said that reality is only what we can demonstrate (the positivist approach) while Einstein said that "the Moon is there even when we are not looking at it." What it comes down to is, when we say a system has a set of attributes or properties, should we regard those attributes and properties as things the system really possesses, or should we regard them as tools we are using to understand the system?

    Realism says that the system would have no idea what to do if it did not have those attributes and properties, but the anti-realist says that the system has no idea what we mean by attributes and properties, they are all our creations and the system does whatever it does just fine without them, we merely imagine the system has them to help us understand that system. The key point in quantum mechanics is that if we adopt the realist approach, we are always looking for hidden variables to be those attributes and properties, but if we adopt the anti-realist approach, we don't expect that the system needs any hidden variables in order to do the things it does.

    This is more a misconception from the pop sci article than the actual issue. If we end up deciding that QM is incompatible with realism, it means that no hidden variable theory that we can stomach will ever be able to recover its predictions. That in turn would mean that quantum systems don't do what they do because of attributes or properties, they just do what they do, and we use attributes and properties to get a good but still incomplete understanding. None of that implies that nothing could exist before there were humans, it just means that we didn't understand quantum systems before there was quantum mechanics, and we might never understand them completely because we like to think about attributes and properties.
     
  12. StevieTNZ

    StevieTNZ 1,047
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    Was just about to post this comment!

    The reference [11] is the article mentioned by bhobba.

    I came across this article: http://iopscience.iop.org/1742-6596/128/1/012017
     
  13. What? That's it? I even thought people thought of that like over a quarter of a century ago. It seemed more obvious even to me that after a certain point in the quantum level that things happen for no predictable reason.
    Also, if there was actually a reason for everything, would that mean we would have to calculate infinite variables since there wouldn't be an "end" or "limit" to how indepthly these attributes go? As in, an atom would be infinitely complex if there were always hidden variables and thus even then we could never get accurate predictions?
     
  14. bhobba

    bhobba 5,092
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    It doesn't - it is possible that a Quantum State exists out there in reality like say an electric field does and wave-function collapse happens by some kind of decoherence process. Trouble is, as far as I know, it hasn't been shown to resolve all the issues. So IMHO realism is still possible - but more work needs to be done. Personally I don't believe a quantum state exists like that.

    Thanks
    Bill
     
  15. Demystifier

    Demystifier 5,176
    Science Advisor

    That paper does not rule out Bohmian mechanics. Instead, it rules out an incorrect interpretation of Bohmian mechanics. See e.g. the first two paragraphs of
    http://xxx.lanl.gov/abs/quant-ph/0305131
    and the published references therein.
     
  16. Demystifier

    Demystifier 5,176
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    That paper does not rule out Bohmian mechanics, because that paper assumes a form of locality (called "non-signalling condition" in the paper) similar to locality in the Bell theorem. In v3 version of the paper, you can see that on page 12, section "REMARKS ON THE NOTION OF LOCALITY". Bohmian mechanics does not obey this locality condition, so is compatible with the results of that paper.
     
  17. Demystifier

    Demystifier 5,176
    Science Advisor

    That "broad class of hidden variables" which are ruled out are actually non-contextual hidden variables. Bohmian hidden variables are contextual, so do not belong to that class. Indeed, Zeilinger et al say explicitly in their original paper that Bohmian mechanics is not ruled out by their results.
     
  18. bhobba

    bhobba 5,092
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    We know that an observation gives a result with 100% certainty - its just we can't predict what that result is. That could be because of some kind of chaotic behavior that in principle is perfectly predictable but in practice cant be - but then again maybe not - we simply do not know.

    If we had a theory of everything where everything happens for a reason that does not lead to any problems or infinite regress as far as I can see.

    Thanks
    Bill
     
  19. Demystifier

    Demystifier 5,176
    Science Advisor

    Bohmian mechanics has the same measurable predictions as standard QM, PROVIDED THAT THE DEGREES OF FREEDOM OF THE MEASURING APPARATUS ARE TAKEN INTO ACCOUNT. This paper does not take into account the degrees of freedom of the measuring apparatus, so does not rule out Bohmian mechanics.
     
  20. Ken G

    Ken G 3,566
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    But note that the form of "realism" that is being talked about is a form that is not supposed to require any measurement at all, so no degrees of freedom in the apparatus are relevant to how "realistic" some system is prior to being measured. Hidden variables are sometimes imagined to be a way to allow a system to be "real" in this sense, so the issue here is, does the Bohmian description of a system, absent of any measurement, allow us to imagine the system is "real", in the sense of having its behavior completely described by its attributes independent from the attributes of any measuring apparatus? Personally, I think the question is meaningless, because I don't think the concept of "behavior of a system" means anything beyond "interaction with measuring devices." But I would not say "reality doesn't exist until we measure it", I would say "the concept of realism relies on the concept of measurement." This is not generally the way the word "realism" is used, but I think the way it is usually used makes no scientific sense. Hence, I don't think the issue of whether BM should be regarded as "realistic" or not is of any importance to its core validity.
     
  21. Ken G

    Ken G 3,566
    Gold Member

    There are two separate issues here. One is why things happen, the other is simply a description of what is happening. The first issue relates to determinism, and the second to realism. Although there are clear connections there (most realists are also determinists, and most anti-realists are also anti-determinists), they are not the same thing. The "realism" being discussed in that article is about whether we can regard systems as having innate properties independent of whether or not they are interacting with a measuring apparatus (we can be vague about what constitutes a measuring apparatus, that's a whole other issue). The issue of determinism is how those properties determine what happens. Most people connect those concepts, but I could imagine a "real" property of doing random things (if we adopt a stochastic interpretation of the wavefunction but also think the wavefunction is a real property, though there appear to be recently discovered problems with this approach), or a "deterministic" behavior that cannot be traced to a property (isolated objects follow inertial paths, regardless of any of their properties).

    So I would say the key issue is not whether or not things have a reason for doing what they do, but rather, do the things themselves have "any idea" what are these "properties" we associate with them (if you follow my drift)? If reality exists in the way we understand it before we measure it, and the measurement just confirms that reality, that is the brand of "realism" that is being talked about. But the pop sci article oversimplifies that terrain by imagining that the possibilities are either that reality "pops into being" when the measurement occurs, or that reality is always there in the same form as the measured version. Neither of those possibilities seem very plausible to me-- much more plausible, and more consistent with quantum mechanics (a la Bohr, anyway), is that reality passes from something that is pretty much inscrutable to us, to something that we can begin to understand, once we pass it through the filter of a measuring apparatus. Is that "realism", or isn't it? The way the word is usually used, no, but I think the way it is usually used is hopelessly naive.
     
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